The present invention relates to 3-D sensing and more particularly concerns a sensor and a method for making range measurements using a TDI array.
In the field of information sensing, machine vision technologies provide valuable information about the environment and about specific objects of interest through close inspection. Many known 3-D data acquisition systems exist that use 3-D sensors based on the triangulation principle. In such systems, a specific known and often fixed pattern of illumination, i.e. structured illumination, is projected from a laser onto an object to be measured. A digital camera, generally based on a charge coupled device (CCD) matrix array, is placed at a known fixed and oblique angle with respect to the light projector. The intersection between the emitted light pattern and the field of view of the digital camera defines the operating range of the 3-D sensor. The position of the illuminated points on the object surface that are imaged on the detector can be computed trigonometrically in order to obtain the sensor/object distance to these points. These point distance measurements are also called range points.
FIG. 1 (prior art) shows such a triangulation based 3-D sensor often called a laser profilometer, where the structured light pattern projected is a laser line. Such profilometers analyze the deformations of the laser line on the object in order to evaluate the depth or range (z-axis) as well as the horizontal position (x-axis) of the object. Generally, the translation of either the profilometer or the object to be scanned by the means of a translation device, often coupled to an encoder, allows the system to obtain the missing (y-axis) position. Consequently, a complete 3-D set of points of the object""s surface can be acquired.
The above-described system is widely used in industrial environments where the objects to be scanned are in the process of being conveyed. In such a scenario, the translation mechanism is the conveyor, usually coupled to an encoder which measures the displacement and speed of the conveyor.
One drawback of existing cameras available for range and 3-D measurements is that manufacturers usually provide such cameras in a limited amount of shapes and sizes. State of the art commercially available 2-D matrix sensors typically have arrays of 640xc3x97480 pixels, which operate at 30 frames per second. This allows the extraction, based on a triangulation setup with a laser line projector, of a 3-D profile of 640 points at 30 Hz. With partial scanning and using a quad-speed camera, 640 point profiles at 350 Hz are possible, and, using specialized high frame rate cameras, 256 point profiles at 900 Hz may be achieved. If high lateral resolution, that is points per profile, is desired (ex: 2048 points per profile), very expensive and slow (maximum frame rate of 30 Hz) 2048xc3x972048 arrays need to be used.
Long detectors are however available in the 2-D imaging industry in the form of TDI devices. The Time Delayed Integration (TDI) principle is a technique used for increasing the sensitivity of standard line array cameras through the use of multiple integration lines or stages (present state of the art allows 96 integration stages). Such a principle of operation can only be applied for imaging moving objects, when the motion of the object is known or can be measured and when the motion of the object is perpendicular to the integration lines of the photodetector array. These conditions are necessary so that the object displacement can be synchronized with the camera""s integration stages. In such a scenario, as the object to be imaged approaches the first line of pixels (first integration stage) of the TDI camera""s photodetector array, this stage xe2x80x9cseesxe2x80x9d the object for a short time period and thus integrates during this time period the image of the corresponding linear portion on the object. As the object moves forward the second integration stage now xe2x80x9cseesxe2x80x9d the same linear object portion that the first stage just imaged. The light equivalent electronic charges integrated by the first stage are then transferred to the second stage and the second stage can now integrate the image of the same linear portion of the object for another short time period. This process is then repeated as the object moves in front of all the integration lines. The line image that is thus outputted from the image sensor is thus equivalent to the sum of all the integration stages. For example, a widely available type of TDI array has 2048 pixels wide photodetectors and has 96 integration stages, which correspond to 96 lines of 2048 pixels on the detector. With this detector, the image that is outputted line by line corresponds to the sum of the 96 previous integration periods.
In an industrial conveyor and web inspection setting, time delayed integration (TDI) cameras are often used for imaging, line by line, objects or materials as they move at high speeds down production lines. These cameras are used because the multiple integration lines on the CCD give these cameras much improved light sensitivity, which is very important when short integration times are necessary for the imaging of the fast moving objects. Also, for industrial applications, it is possible to place the TDI camera perpendicular to the object motion and to synchronize the integration stages with the object displacement using encoders.
While TDI cameras are frequently used for imaging in web type industrial inspection applications, to date, they have not been used as the detection means for range sensors. While it seems that a TDI line array can readily replace traditional line array detectors commonly used as detection means for single point type laser and flying spot laser scanning 3-D sensors, this in fact has not been the case. One reason is that the performance gain, in terms of speed or resolution of these types of 3-D sensors, would not be increased by the use of a TDI sensor and with the added difficulty of synchronizing the motion of the object with the sensor, manufacturers have not adopted this approach. For 3-D sensors such as laser profiling sensors that use 2-D matrix type photodetector arrays it might even seem impossible to use a TDI type detector array as a detection means because, for such a sensor, the information necessary to determine the position (elevation) of the incident light reflected from the object surface and imaged on the detector matrix is eliminated as the TDI sensor proceeds through its integration procedure.
There is therefore a need for an apparatus and method allowing the use of a TDI array for range 3-D applications.
It is an object of the present invention to provide a sensor adapted to obtain range points of a target object using a TDI device.
It is a further object of the present invention to provide a method for range measurements of a target object using a TDI array.
It is a preferable object of the present invention to provide such a sensor and such a method adapted for industrial applications.
Accordingly, a first aspect of the present invention provides a sensor for measuring a range to a target object by triangulation, using a TDI device.
The sensor first includes a light source, which generates a light beam for projection towards the target objet and for reflection thereon. A plurality of photodetector arrays, defining the TDI device, are disposed contiguously in a linear sequence, the TDI device being positioned to detect the reflection of the light beam on the target object across a plurality of these photodetector arrays.
The TDI device is operable over an acquisition cycle composed of a number of integration periods. Each photodetector array accumulates electronic charges representative of light incident thereon during each integration period. Each of the photodetector arrays transfers all electronic charges therein to a next photodetector array along the linear sequence after each integration period. The TDI device has an output for transmitting all electronic charges from a last photodetector array along the linear sequence after each integration period.
The sensor also includes restricting means for restricting an exposition of the TDI device to the reflection of the light beam on the target object to a selected integration period of the acquisition cycle.
Finally, processing means are provided for receiving the electronic charges from the output of the TDI device after each integration period of the acquisition cycle, and processing the electronic charges of the acquisition cycle to obtain therefrom the range to the target object.
In accordance with a second aspect of the present invention, there is provided a sensing apparatus for measuring a range to a target object over a plurality of regions thereof by triangulation, using a TDI device.
The sensing apparatus includes a light source generating a light beam for projection towards an exposition area, and positioning means for successively positioning each of the regions of the target object within this exposition area.
A plurality of photodetector arrays are provided and disposed contiguously in a linear sequence, these photodetector arrays defining the TDI device. The TDI device is positioned to detect the reflection of the light beam on a region of the target object, which extends in the exposition area across a plurality of the photodetector arrays. The TDI device is operable over a plurality of acquisition cycles, each composed of a number of integration periods. Each photodetector array accumulates electronic charges representative of light incident thereon during each integration period, and each of said photodetector arrays transfers all electronic charges therein to a next photodetector array along the linear sequence after each integration period. The TDI device has an output for transmitting all electronic charges from a last photodetector array along the linear sequence after each integration period.
The sensing apparatus also includes restricting means for restricting an exposition of the TDI device to the reflection of the light beam on each region of the target object to a selected integration period of each acquisition cycle. Processing means are also provided for receiving the electronic charges from the output of the TDI device after each integration period of each acquisition cycle, and processing the electronic charges of each acquisition cycle to obtain therefrom the range to the region of the target object during the selected integration period of the acquisition cycle.
In accordance with another aspect of the invention, there is also provided a method for measuring by triangulation a range to a target object extending in an exposition area using a TDI device. This method includes the following steps:
a) generating a light beam, and projecting this light beam towards the exposition area for reflection on the target object;
b) disposing a plurality of detector arrays, defining said TDI device, contiguously in a linear sequence, the TDI device being positioned to detect the reflection of the light beam on the target object within the exposition area across a plurality of the photodetector arrays. The TDI device has, in operation, an acquisition cycle composed of a number of integration periods, each photodetector array accumulating electronic charges representative of light incident thereon during each integration period. Each of the photodetector arrays transfers all electronic charges therein to a next photodetector array along the linear sequence after each integration period. The TDI device has an output for transmitting all electronic charges from a last photodetector array along the linear sequence after each integration period;
c) restricting an exposition of the TDI device to the reflection of the light beam on the target object to a selected integration period of the exposition cycle; and
d) receiving the electronic charges from the output of the TDI device after each integration period of the acquisition cycle and processing the electronic charges of the acquisition cycle to obtain therefrom the range to the target object.
The present invention also provides, in accordance with yet another aspect thereof, a method for measuring by triangulation a range to a target object over a plurality of regions thereof using a TDI device, comprising the steps of:
a) generating a light beam and projecting the same towards an exposition area;
b) successively positioning each of the regions of said target object within the exposition area;
c) disposing a plurality of detector arrays, defining said TDI device, contiguously in a linear sequence, said TDI device being positioned to detect the reflection of the light beam on the region of the target object within the exposition area across a plurality of the photodetector arrays, said TDI device having, in operation, a plurality of acquisition cycles each composed of a number of integration periods, each photodetector array accumulating electronic charges representative of light incident thereon during each integration period, each of said photodetector arrays transferring all electronic charges therein to a next photodetector array along said linear sequence after each integration period, said TDI device having an output for transmitting all electronic charges from a last photodetector array along said linear sequence after each integration period of each acquisition cycle;
d) restricting an exposition of the TDI device to the reflection of the light beam on each region of the target object to a selected integration period of a corresponding acquisition cycle; and
e) receiving the electronic charges from the output of the TDI device after each integration period of each acquisition cycle and processing the electronic charges each of the acquisition cycle to obtain therefrom the range to the corresponding region of the target object.
Advantageously, the use of a TDI device in the place of matrix type photodetector arrays as detection means for 3-D triangulation based sensors allows certain performance gains. The main performance gain is in the number of 3D points per profile combined with a high profile rate that can be achieved using these detectors. Using relatively inexpensive and standard commercially available TDI devices in accordance with the present invention, it is possible to obtain a 3-D profile of 2048 points at a rate of 864 Hz. and using the partial scanning technique described below, 2048 points at a rate of 1728 Hz can be achieved.
Further advantages and features of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.